Steam assisted gravity drainage (SAGD), is a well-known technique for recovery of oil from the tar sands. As the name implies, the technique uses steam, often injected at very high pressures and temperatures, to recover hydrocarbons in situ. In a typical SAGD extraction, the steam is injected into the formation from a generally horizontal injection well and recovered from a lower parallel-running generally horizontal production well. An extraction chamber is developed, first with communication between the wells and eventually up and around the well pair. As the steam flows towards the perimeter of the chamber, it encounters lower temperatures. These temperatures result in a condensation of the steam and then a subsequent flow of hot water that drains downwardly. In this way heat is transferred to the bitumen, causing the bitumen to warm up to the point of melting or flowing. The mobilized bitumen also drains downwardly and then the liquid water and bitumen are recovered from the formation through the production well located near the bottom of the chamber. As the mobilized bitumen drains down, fresh bitumen becomes exposed at an extraction interface that is subsequently heated by the ongoing steam condensation. The continuous drainage of bitumen from the sand results in the steam filled extraction (i.e. bitumen depleted) chamber growing over time. This chamber is called a gravity drainage chamber.
To ensure that the steam vapour does not short circuit directly from the injection well to the production well, the chamber is typically operated with what is called steam trap control. Steam trap control simply means that a liquid head of warmed bitumen and water is maintained above the production well, to ensure that the steam vapour cannot short circuit directly from the injection well into the production well, thereby bypassing the chamber to a large degree and failing to deliver heat to the bitumen at the extraction interface.
Steam trap control is implemented by restricting the fluid production from the production well to ensure that the production well is always immersed in liquid water and bitumen. Steam trap control thus tries to prevent any vapour production by only allowing liquids to be removed from the chamber. Steam trap control is often implemented by trying to achieve a target subcool value. The subcool refers to the temperature (i.e. degrees Centigrade) of the produced fluid below the thermodynamic condensation temperature at the chamber pressure. SAGD operators typically try to maintain fluid temperatures in the range of 5 C to more than 40 C below the condensation temperature of the steam in the chamber to minimize the amount of steam vapour short-circuiting from the production well.
SAGD is a field proven technology, but has low profit margins and huge environmental costs principally due to the tremendous amount of energy and water required to create the steam used in the process. Steam extraction produces large amounts of greenhouse gas emissions (approximately 250 pounds of CO2 per barrel of bitumen) since fuel must be burned to produce the steam. Any way of reducing the energy requirement to extract the bitumen is both economically and environmentally desirable.